1. Field of the Invention
The invention relates to self-adhesive, addition-crosslinking silicone compositions, a process for producing addition-crosslinked silicone elastomers, and composite materials.
2. Background Art
It is known that the adhesion of addition-crosslinked silicone elastomers to numerous substrates, such as plastics, metals, and glasses, is low, and frequently even spontaneous detachment of the silicone elastomer from the substrate is observed. Since numerous applications attach critical importance to a firm and lasting substrate adhesion of the silicone elastomer, however, a multiplicity of specific measures have been proposed for achieving such a firm bond between substrate and silicone elastomer.
In principle, the adhesion of the silicone elastomer/substrate assembly can be increased by appropriately modifying the chemical and/or physical nature of the substrate and/or of the substrate surface, prior to the application of the addition-crosslinking silicone elastomer composition. This can be done, for example, by pretreating the substrate surface with adhesion-promoting additives (known as primers), by plasma treatment of the substrate surface, by mixing specific additives into the substrate material, by specifically adjusting the morphology of the substrate, and by increasing the surface roughness. A disadvantage to all of these measures is that additional process steps become necessary or specific requirements must be imposed on the nature of the substrate.
Furthermore, the adhesion of addition-crosslinking silicone elastomers to various substrates can be improved by means of one or more additives which are incorporated into the uncrosslinked silicone compound. Such additives include compounds containing highly reactive functional groups such as alkoxy, epoxy, carboxyl, and amino groups, for example, these groups generally being selected such that the adhesion promoter has the capacity to react both with the substrate and with a silicone elastomer constituent. Although it is possible in some cases, by means of such adhesion promoters, to forego the pretreatment of the substrate, the adhesion achieved nevertheless frequently fails to meet the requirements imposed. Additionally, increasing the adhesion by means of higher levels of these adhesion promoters is possible only to a limited extent, since the highly reactive groups they contain then increasing adversely impact on properties such as storage stability, crosslinking characteristics (inhibition), and toxicological acceptability, for example.
Efforts have therefore been made to minimize the amount of adhesion promoters. For example, European published application EP 0 875 536 A2 describes a self-adhesive addition-crosslinking silicone rubber blend wherein the SiH crosslinker (a) characteristically contains at least 20 SiH groups, the remaining radicals being aliphatically saturated hydrocarbon radicals, an epoxy-functional alkoxysilane and/or alkoxysiloxane d) is present, and optionally, a peroxide g) is present. Particular preference is given in that context to the use of glycidyloxypropyltrimethoxysilane (Glymo). The silicone rubber blend described in patent EP 0 875 536 A2 is particularly suitable for the production of composite moldings which are composed of a silicone elastomer and an organic polymer. The composition described therein, however, has the disadvantage that adequate adhesion is obtainable only when using very SiH-rich crosslinkers, having on average at least 20 SiH groups per molecule. Indeed, in the examples therein, crosslinkers having 30 SiH groups per molecule are used. The use of crosslinkers of such high functionality considerably lessens the storage stability of addition-crosslinking silicone rubber mixtures: that is, the fluidity is massively impaired, proceeding as far as the stiffening of the compound, as a result of which the proper processing of the compound, for example by injection molding, is no longer possible. Furthermore, in order to obtain a high level of adhesion, it is necessary to use relatively large amounts of epoxy-functional alkoxysilanes or siloxanes, thereby considerably reducing the crosslinking rate. Although this reduction can be partly compensated through use of a peroxide, as described in EP 0 875 536 A2, it is nevertheless the case that the only peroxides suitable for this purpose, due to the low crosslinking temperatures required (softening of the organic polymer), are peroxides having a low initiation temperature, such as the 2,4-dichlorobenzoyl peroxide previously mentioned, and peroxides of this kind on the one hand are very objectionable toxicologically, owing to the cleavage products and secondary products that are liberated (PCB problem), and, on the other hand, further impair the storage stability of the compound. The disadvantages resulting from the use of epoxy-functional alkoxysilanes and siloxanes are the elimination of alcohol group(s), the use of reactive and polar groups, and, in the case of the functional alkoxysilanes, the problems of effluorescence and exudation. The elimination of the alcohol may, on one hand, be detrimental to effective adhesion since the alcohol accumulates at the surface of the silicone and hence at the face toward the substrate, thereby impairing contact between silicone and surface, and on the other hand, preference is given to using methoxysilanes, which release methanol, which is classed as toxic. Moreover, the liberation of volatile cleavage products (alcohol elimination) is accompanied by a not inconsiderable contraction of the silicone elastomer, which in general is undesirable.
The use of the reactive epoxide and/or alkoxy groups results, on account of their polarity, in a thixotropic silicone rubber composition, which may no longer be fluid, and, secondly, the reactive groups may react during storage, so as to no longer be available for the development of adhesion. If functional alkoxysilanes are used, then the silane molecules that have not been consumed by reaction in the course of crosslinking gradually migrate to the surface of the silicone elastomer, where they undergo hydrolysis and condensation to form silsesquioxanes, this formation being manifested as effluorescence on the surface and leading to a clouding of the elastomer. Moreover, because they migrate to the surface, they cause mold deposits in the course of injection molding, which gives rise to increased cleaning effort and longer idle times of the injection molding unit.
German patent application DE 102 04 893 A1 describes self-adhesive addition-crosslinking silicone mixtures which characteristically comprise at least one organohydrosiloxane crosslinker b) which is required to meet the conditions: i) one of the organohydrosiloxanes contains more than 7 mmol SiH/g; ii) one of the organohydrosiloxanes contains at least one aromatic group in the molecule; and iii), i) and ii) can be contained in the same organohydrosiloxane or in different organohydrosiloxanes. In spite of this specific crosslinker b), effective adhesion still requires at least one alkoxysilane and/or alkoxysiloxane containing in each case at least one epoxy group as well. Here too, therefore, there are the same problems as already described with the alkoxy/epoxy adhesion promoters.
European published application EP 0 686 671 A2 describes a self-adhesive addition-crosslinking material which uses no specific adhesion promoters, since the adhesion-promoting constituent either is an organohydropolysiloxane which possesses on average per molecule at least two SiH groups, at least 12 mol % of whose monovalent Si-bonded radicals are hydrocarbon radicals having an aromatic ring (it is expressly indicated that satisfactory adhesion is not achieved if the amount of the organic radical that carries aromatic ring is below 12 mol %), or is a compound that possesses on average, per molecule, at least one SiH group and contains a group consisting of two aromatic rings, the two aromatic rings being separated from one another by R13R14Si, R13R14SiO—, —OR13R14SiO— or —R13R14SiOR13R14Si—, where the radicals R13 and R14 represent monovalent hydrocarbon radicals. The adhesion-promoting constituent, therefore, can be the same as the crosslinker of the silicone elastomer compound. This composition produces effective adhesion to organic polymers, particularly acrylonitrile-butadiene-styrene (ABS) copolymer, but at the same time exhibits great ease of demolding from metals. The high level of radicals containing aromatic rings in the SiH-containing, adhesion-promoting constituent, of more than 12 mol %, however, means that there is a considerable degree of incompatibility with the other constituents of the addition-crosslinking silicone elastomer compound. This leads on the one hand to partial separation (exudation) during storage, which necessitates repeated homogenization of the component containing this constituent prior to use. This incompatibility, which is already apparent from a milky cloudiness on the part of the uncrosslinked compound, is also manifested in a significantly reduced transparency on the part of the silicone elastomer parts produced from it. If the adhesion-promoting constituent functions also as the crosslinker for the silicone elastomer composition, the incompatibility leads to vulcanization defects, which leads in turn to inhomogeneous network formation and inadequate mechanical properties on the part of the vulcanizate. In order to avoid these vulcanization defects, it is necessary, in addition to the adhesion-promoting SiH-containing constituent, to use an SiH-containing crosslinker which is fully compatible with the silicone elastomer compound, although this results in other disadvantages, such as increased compression set values and increased exudation propensity of the adhesion-promoting constituent. The high level of radicals containing aromatic rings in the SiH-containing, adhesion-promoting constituent, of more than 12 mol %, also gives rise to a considerable structural viscosity and thixotropy in the silicone elastomer compound, which is undesirable in numerous applications, an example being the injection molding of liquid silicone rubber.
European patent EP 0 728 825 B1 describes self-adhesive silicone rubbers which comprise, as crosslinkers, R3Si(OSi(R)H)nOSiR3, (OSi(R)H)n— or R4-1 Si(OSi(R)2H)1, where n is at least 3 and 1=3 or 4, and a specific adhesion promoter. This adhesion promoter is a molecule which contains at least one aliphatically unsaturated group and at least two phenylene groups.
European patent EP 1 106 662 B1 describes self-adhesive addition-crosslinking materials which allow effective adhesion as a result of a synergistic effect obtained from the combination of a specific Si—H crosslinker (B) and an organosilicon compound (C) with epoxide groups and hydrolyzable groups. Although that patent does offer a solution in terms of the crosslinker, it still exhibits disadvantages in respect of the adhesion-promoting additive (C).
In summary it can be stated that none of the conventional addition-crosslinking silicone elastomer compositions do justice to the requirements that are imposed on a self-adhesive silicone elastomer material that is to be used particularly for the production of composite moldings or for the encapsulation of electrical/electronic parts, said requirements including:
a) high fluidity (very little thixotropy, if any) and high storage stability,
b) high crosslinking rate at relatively low temperatures,
c) high adhesion to organic polymers and metals,
d) high adhesion even after storage of the uncrosslinked components and subsequent processing,
e) high adhesion of the composite even after its storage,
f) toxicological acceptability and suitability for use in the food or near-food sector,
g) high level of service properties (transparency, noncorrosiveness, profile of mechanical properties),
h) minimal, if any, deposits in the injection mold, and
i) minimal contraction of elastomer parts on injection molding.